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Mitogen-activated protein kinase (MAPK) cascades play a key role in plant growth and development as well as in biotic and abiotic stress responses. They are classified according to their sequence homology into four major groups (A-D). A large amount of information about MAPKs in groups A and B is available but few data of the C group have been reported. In this study, a C1 subgroup MAP kinase cDNA, PsMPK2, was isolated from Pisum sativum. PsMPK2 is expressed in vegetative (root and leaf) and reproductive (stamen, pistil and fruit) organs. Expression of PsMPK2 in Arabidopsis thaliana shows that mechanical injury and other stress signals as abscisic acid, jasmonic acid and hydrogen peroxide increase its kinase activity, extending previous results indicating that C1 subgroup MAPKs may be involved in the response to stress.

Mitogen-activated protein kinase (MAPK) cascades play an important role in mediating stress responses in plants. In Arabidopsis, 20 MAPKs have been identified and classified into four major groups (A-D). Little is known about the role of group C MAPKs. We have studied the activation of Arabidopsis subgroup C1 MAPKs (AtMPK1/AtMPK2) in response to mechanical injury. An increase in their kinase activity was detected in response to wounding that was blocked by cycloheximide. Jasmonic acid (JA) activated AtMPK1/AtMPK2 in the absence of wounding. Wound and JA-induction of AtMPK1/2 kinase activity was not prevented in the JA-insensitive coi1 mutant. Other stress signals, such as abscisic acid (ABA) and hydrogen peroxide, activated AtMPK1/2. This report shows for the first time that regulation of AtMPK1/2 kinase activity in Arabidopsis might be under the control of signals involved in different kinds of stress.

Early endocytic membrane traffic is regulated by the small GTPase Rab5, which cycles between GTP- and GDP-bound states as well as between membrane and cytosol. The latter cycle depends on GDI, which functions as a Rab vehicle in the aqueous environment of the cytosol. Here, we report that formation of the GDI:Rab5 complex is stimulated by a cytosolic factor that we purified and then identified as p38 MAPK. We find that p38 regulates GDI in the cytosolic cycle of Rab5 and modulates endocytosis in vivo. Our observations reveal the existence of a cross-talk between endocytosis and the p38-dependent stress response, thus providing molecular evidence that endocytosis can be regulated by the environment.

PsMAPK3, a new MAP kinase cDNA, was cloned from ovaries of Pisum sativum L. Expression of PsMAPK3 is at low basal levels in unpollinated ovaries but it is rapidly induced by gibberellic acid (peak at 30 min) and 6-benzyladenine (peak at 45 min). Both treatments promoted the development of a parthenocarpic fruit. In situ hybridization localized PsMAPK3 mRNA in ovules. The transcript was additionally detected in the mesocarp when it is expanding in response to the treatments. These observations suggest that gibberellins and cytokinins regulate PsMAPK3 mRNA levels in pea ovary shortly after fruit set is induced.